Structure and method for vertical tunneling field effect transistor with leveled source and drain

ABSTRACT

The present disclosure provides one embodiment of a semiconductor structure. The semiconductor structure includes a semiconductor substrate having a first region and a second region; a first semiconductor mesa formed on the semiconductor substrate within the first region; a second semiconductor mesa formed on the semiconductor substrate within the second region; and a field effect transistor (FET) formed on the semiconductor substrate. The FET includes a first doped feature of a first conductivity type formed in a top portion of the first semiconductor mesa; a second doped feature of a second conductivity type formed in a bottom portion of the first semiconductor mesa, the second semiconductor mesa, and a portion of the semiconductor substrate between the first and second semiconductor mesas; a channel in a middle portion of the first semiconductor mesa and interposed between the source and drain; and a gate formed on sidewall of the first semiconductor mesa.

BACKGROUND

The scaling of conventional complementary metal-oxide-semiconductorfield effect transistor (CMOSFET) faces challenges of rapid increase inpower consumption. Tunnel field effect transistor (TFET) is a promisingcandidate enabling further scaling of power supply voltage withoutincrease of off-state leakage current due to its sub-60 mV/decsubthreshold swing. However, in a vertical TFET, the source and drainare at different horizontal levels, which present various issues. Forexample, the contacts to the source and drain face more challenge due tothe height difference.

Accordingly, there is a need for a structure having vertical TFET deviceand a method making the same to address above concerns.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1-11 are sectional views of a semiconductor structure having atunnel field effect transistor (TFET) structure at various fabricationstages constructed according to one or more embodiments.

FIG. 12 is a flowchart of a method to form the semiconductor structureof FIG. 11 constructed according to one embodiment.

FIG. 13 is a sectional view of a semiconductor structure having a TFETstructure and a capacitor constructed according to another embodiment.

FIG. 14 is a sectional view of a semiconductor structure having aresistor constructed according to another embodiment.

FIG. 15 is a sectional view of a semiconductor structure having aresistor constructed according to another embodiment.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.Moreover, the formation of a first feature over or on a second featurein the description that follows may include embodiments in which thefirst and second features are formed in direct contact, and may alsoinclude embodiments in which additional features may be formedinterposing the first and second features, such that the first andsecond features may not be in direct contact.

FIGS. 1-11 are sectional views of a semiconductor structure 100 atvarious fabrication stages constructed according to one or moreembodiment. The semiconductor structure 100 includes one or more tunnelfield effect transistor (TFET). In furtherance of the embodiment, theTFET has a vertical structure wherein the channel is verticallyconfigured and interposed between the source and drain. FIG. 12 is aflowchart of a method 200 to form the semiconductor structure 100constructed according to one or more embodiment. The semiconductorstructure 100 and the method 200 making the same are collectivelydescribed with reference to FIGS. 1-12.

Referring to FIG. 1, the semiconductor structure 100 includes asemiconductor substrate 110 of a first semiconductor material. In thepresent embodiment, the first semiconductor material is silicon.Alternatively, the first semiconductor material may include other propersemiconductor material. In one embodiment, the semiconductor substrate110 includes a buried dielectric material layer for isolation formed bya proper technology, such as a technology referred to as separation byimplanted oxygen (SIMOX). In some embodiments, the substrate 110 may bea semiconductor on insulator, such as silicon on insulator (SOI).

The semiconductor substrate 110 includes a first region 112 and a secondregion 114. The semiconductor substrate 110 includes a tunnel fieldeffect transistor (TFET) formed in the first second regions. In thepresent embodiment, the TFET is a vertical TFET where the channel of theTFET is along a direction perpendicular to the top surface of thesemiconductor substrate 110.

Referring to FIGS. 1 and 12, the method 200 begins at operation 202 byforming a patterned hard mask 116 to define areas for a firstsemiconductor mesa on the semiconductor substrate 110 within the firstregion 112 and a second semiconductor mesa on the semiconductorsubstrate 110 within the first region 112. Specifically, the patternedhard mask 116 includes a first feature in the first region 112 and asecond feature in the second region 114. Each feature has a geometrydefining the geometry of the corresponding semiconductor mesa. In thepresent embodiment, each feature of the patterned hard mask 116 has around shape with a diameter D, which depends on the designed size of thecorresponding semiconductor mesa and further depends on the fabricationbias. In one example, the diameter D ranges between about 10 nm andabout 50 nm.

The patterned hard mask 116 includes a dielectric material with etchselectivity to the semiconductor substrate 110. In the presentembodiment, the patterned hard mask 116 includes silicon nitride (SiN).In other embodiments, the patterned hard mask 116 alternatively includesother suitable material, such as silicon oxynitride or silicon carbide.

In one embodiment, the patterned hard mask 116 is formed by a procedureincluding deposition, lithography process and etching. In furtherance ofthe embodiment, the formation of the patterned hard mask 116 includesdepositing a hard mask layer by a suitable technique, such as chemicalvapor deposition (CVD); forming a patterned photoresist layer 118 on thehard mask layer using a lithography process; etching the hard mask layerto form the patterned hard mask 116 using the patterned photoresistlayer 118 as an etch mask; and thereafter removing the patternedphotoresist layer 118 by a suitable technique, such as wet stripping orplasma ashing. In one embodiment, the lithography process includesforming a photoresist layer by spin-on coating; exposing the photoresistlayer using an exposure energy, such as ultraviolet (UV) light, anddeveloping the exposed photoresist layer to form the patternedphotoresist layer using a developing chemical. In another example, thelithography process includes spin-on coating, soft baking, exposing,post-exposure baking, developing and hard baking. In other embodiment,the lithography process to form the patterned photoresist layer 118 mayalternatively use other technique, such as e-beam lithography, masklesspatterning or molecular print.

Referring to FIGS. 2 and 12, the method 200 includes an operation 204 byselectively recessing the semiconductor substrate to form semiconductormesas 120, especially the first semiconductor mesa 120A in the firstregion 112 and the second semiconductor mesa 120B in the second region114.

In the present embodiment, the first and second semiconductor mesas havea coplanar top surface. The semiconductor mesas (120A and 120B) have aheight “H1” as a vertical dimension relative to the top surface 121 ofthe semiconductor substrate 110. In one example, the recess depth rangesbetween about 50 nm and about 200 nm. Therefore, the height H1 of thesemiconductor mesas 120 is in the same range for this example.

The first and second semiconductor mesas are simultaneously formed in asame procedure. In the present embodiment, an etch process is applied toselectively etch the semiconductor substrate 116 using the patternedhard mask 116 as an etch mask. For example, the etch process includes adry etch to etch silicon of the semiconductor substrate 110. In oneembodiment, the etch process is tuned to form the semiconductor mesa120A (120B as well) having a sidewall profile in a trapezoidal shape.Particularly, the sidewall profile of the each semiconductor mesa has atilting angle less than 90° and greater than 45°, where the tiltingangle is measured relative to the top surface 121 of the semiconductorsubstrate 110. Thus formed the semiconductor mesa (120A or 120B) has abetter fabrication benefits during the subsequent process steps, such asdeposition and/or etch.

Referring to FIGS. 3 and 12, the method 200 includes an operation 206 byforming a plurality of isolation features 122 in the semiconductorsubstrate 110. In the present embodiment, the isolation features 122 areshallow trench isolation (STI) features 122. The STI features 122 areformed in the semiconductor substrate 110 and define various activeregions. In this case, the first region 112 and the second region 114are within a same active region. Furthermore, the top surface 121 of thesemiconductor substrate 110 and top surfaces of the STI features 112 arecoplanar at the present fabrication stage.

Since the presence of the semiconductor mesas 120, the formation of theSTI features 122 is designed to avoid the damage to the semiconductormesas 120.

In one embodiment, the formation of the STI features 122 includes:forming a hard mask with openings that define the regions for STIfeatures; etching the semiconductor substrate 110 through the openingsof the hard mask to form trenches; depositing dielectric material tofill in the trenches; performing a chemical mechanical polishing (CMP)process to remove excessive dielectric material above the semiconductormesa 120; and then selectively etching back the dielectric material tothe top surface of the semiconductor substrate 110, resulting in the STIfeatures 122. In the CMP process, the patterned hard mask 116 may serveas a polishing stop layer such that the CMP process properly stops onthe patterned hard mask 116. In the etch-back process, the patternedhard mask 116 may serve as an etch mask to further protect thesemiconductor mesas 120 from loss.

In another embodiment, the STI features 122 are formed before theformation of the semiconductor mesas 120. In furtherance of theembodiment, the formation of the STI features 122 includes: forming ahard mask with openings that define the regions for STI features;etching the semiconductor substrate 110 through the openings of the hardmask to form deep trenches; depositing dielectric material to fill inthe trenches; and performing a CMP process to remove excessivedielectric material above the semiconductor substrate 110, resulting indeep trench isolation features. Thereafter, the operations 202 and 204are performed to form the patterned hard mask 116 and to form thesemiconductor mesas 120, respectively. However, in the operation 204 torecess the semiconductor substrate 110 by an etch process, the etchprocess is designed to recess both the semiconductor material (siliconin the present embodiment) of the semiconductor substrate 110 and thedielectric material of the deep trench isolation features. Thus, theupper portions of the deep trench isolation features are removed,resulting in shallow trench isolation features 122. The heightdifference between the deep trench isolation features and the STIfeatures 122 is about the height H1 of the semiconductor mesa 120.

In another embodiment, the deposition of the dielectric materialincludes thermal oxidation of the trenches and then filling in thetrenches by the dielectric material, such as silicon oxide, by CVD. Inone example, the CVD process to fill in the trenches includes highdensity plasma CVD (HDPCVD).

Referring to FIGS. 4 and 12, the method 200 includes an operation 208 toform the drain 126 of the TFET by a first ion implantation process 124.The drain 126 is formed in the bottom portion of the first semiconductormesa 120, the bottom portion of the first semiconductor mesa 120 and aportion of the semiconductor substrate 110 below the top surface 121.The drain 126 is a continuous doped feature extending from the firstsemiconductor mesa 120A to the second semiconductor mesa 120B throughthe semiconductor substrate 110. In the present embodiment, the drain126 includes a n-type dopant (such as phosphorous) when the TFET isn-type or a p-type dopant (such as boron) when the TFET is p-type.

In one embodiment, the operation 208 includes depositing a screeninglayer 128 on the semiconductor substrate 110 and the semiconductor mesas120; and performing a selective implantation to the semiconductorsubstrate 110 and the semiconductor mesas 120. The screening layer 128is used for implantation screening and elimination of the channelingeffect during the implantation.

Particularly, the selective implantation includes forming a patternedphotoresist layer on the semiconductor substrate 110, performing the ionimplantation process 124 using the patterned photoresist layer as animplantation mask, and removing the patterned photoresist layerthereafter by wet stripping or plasma ashing. The patterned photoresistlayer covers other regions not intended for the ion implantation process124. The patterned photoresist layer is formed by a lithography processas described above.

The drain 126 formed by the ion implantation 124 is further annealed foractivation by an annealing process. The annealing process is implementedright after the ion implantation 124 in the operation 208 or isalternatively implemented after the formation of other doped featuresfor collective activation. In one embodiment, the annealing processincludes rapid thermal annealing (RTA). In other embodiments, theannealing process alternatively includes laser annealing, spikeannealing, million second anneal (MSA) or other suitable annealingtechnique.

Referring to FIGS. 5 and 12, the method 200 includes an operation 210 toform a TFET isolation layer 130. The TFET isolation layer 130 providesisolation function to and properly configures various features of theTFET. For examples, the gate is properly aligned with the channel, notdirectly formed on the semiconductor substrate 110, and is substantiallyoff from the drain.

The TFET isolation layer 130 includes a dielectric material, such assilicon oxide in the present example. The TFET isolation layer 130 mayalternatively include other suitable dielectric material. The TFETisolation layer 130 is disposed on the semiconductor substrate 110.Particularly, the thickness T1 of the TFET isolation layer 130 is chosensuch that the subsequent formed gate can be properly configured with thechannel and the drain. As illustrated in FIG. 5, “H2” is the height ofthe drain 126 measured from the top surface of the semiconductorsubstrate 110 up to the top surface of the drain 126. The thickness T1of the TFET isolation layer 130 is chosen such that T1 is little less H1as T1<H1, to has a small overlap between the gate and drain, and tofurther ensure that the gate completely couples with the channel.

In one embodiment, the operation 210 includes removing the screen layer128 by an etch process (such as a wet etch); and forming a dielectricmaterial layer (such as silicon oxide in the present embodiment) on thesemiconductor substrate 110. In one embodiment, the forming of thedielectric material layer includes depositing a dielectric material,performing a CMP process to remove a portion of the dielectric materialabove the semiconductor mesas 120, and etch back the dielectricmaterial. In another embodiment, the dielectric material layer isselectively removed from other regions by a procedure including forminga patterned photoresist layer on the semiconductor substrate 110,performing an etch process to the dielectric material layer using thepatterned photoresist layer as an etch mask, and removing the patternedphotoresist layer thereafter by wet stripping or plasma ashing.

Referring to FIGS. 6 and 12, the method 200 includes an operation 212 toform gate stack on the semiconductor substrate 110. The formation of thegate stack includes forming gate material layers and patterning the gatematerial layers to form the gate stack.

The gate material layers are formed on the first semiconductor mesa 120Aand on the TFET isolation layer 130. Especially, the gate materiallayers are formed on sidewalls of the first semiconductor mesa 120A andon the top surface thereof as well. In the present case, the gatematerial layers are disposed on the patterned hard mask 116.

The gate material layers include gate a dielectric material layer 134and a gate electrode layer 136. In the present embodiment, the gatematerial layers include high k dielectric material and metal, therefore,referred to as high k metal gate. In one embodiment, the gate dielectricmaterial layer 134 includes an interfacial layer (such as silicon oxide)and a high k dielectric material layer. A high k dielectric material isa dielectric material having a dielectric constant greater than that ofthermal silicon oxide. For example, a high k dielectric materialincludes hafnium oxide (HfO) or other suitable metal oxide. The gateelectrode layer 136 includes a metal (or metal alloy) layer and mayfurther include a polycrystalline silicon (polysilicon) layer on themetal layer.

The operation 212 includes depositing various gate materials on thesemiconductor substrate, specifically on the TFET isolation feature 130and the semiconductor mesas 120. Especially as described in oneembodiment where the semiconductor mesa 120A has a trapezoidal profile,it is beneficial for depositions of various gate materials. In oneembodiment, the formation of the interfacial layer (silicon oxide in thepresent example) includes thermal oxidation, ALD, CVD or other suitabletechnology. In another embodiment, the formation of the high kdielectric material layer includes ALD, metalorganic CVD (MOCVD),physical vapor deposition (PVD), or other suitable technology. In yetanother embodiment, the formation of the metal layer includes PVD,plating, or other suitable technology. In yet another embodiment, theformation of the polysilicon layer includes CVD or other suitabletechnology.

The operation 212 also includes patterning the gate material layersincluding the gate dielectric material layer 134 and the gate electrodelayer 136, resulting in a gate material stack in the first region 112.The material stack includes a first portion on the top of the firstsemiconductor mesa 120A, a second portion on the sidewall of the firstsemiconductor mesa 120A, and a third portion on the top surface of theTFET isolation layer 130. The third portion of the material stack ishorizontally extended on the TFET isolation layer 130.

In one embodiment, the patterning of the gate material layers includesforming a patterned photoresist layer 142 on the gate material layers,performing an etch process to the gate material layers using thepatterned photoresist layer 142 as an etch mask, and removing thepatterned photoresist layer 142 thereafter by wet stripping or plasmaashing. In one example, the etch process includes more than one etchsteps using different etchants to etch respective materials in the gatematerial layers. Each etchant is designed to effectively etch therespective material. The patterned photoresist layer 142 is formed by alithography process. The patterned photoresist layer 142 covers thesemiconductor substrate 110 in the first region 112, as illustrated inFIG. 6.

Referring to FIGS. 7 and 12, the method 200 may include an operation 214to form a TFET isolation layer 150 on the semiconductor substrate 110.The TFET isolation layer 150 provides isolation function to and properlyconfigures various features of the TFET. For examples, the source of theTFET is properly configured thereby.

The TFET isolation layer 150 includes a dielectric material, such assilicon oxide in the present example. The TFET isolation layer 150 mayalternatively include other suitable dielectric material, such as low kdielectric material. The TFET isolation layer 150 is disposed on thesemiconductor substrate 110, the TFET isolation layer 130 and the gatematerial stack. Particularly according to the present embodiment, thethickness of the TFET isolation layer 150 is chosen such that aremaining isolation thickness T2 is about ⅓ of the total vertical heightof the semiconductor mesa 120. The remaining isolation height T2 is avertical dimension measured from the top surface of the horizontalportion of the material stack up to the top surface of the TFETisolation layer 150. The length of the channel is associated with theremaining isolation thickness T2 and is determined thereby.

In one embodiment, the operation 214 includes deposition of thedielectric material (silicon oxide in the present example), performing aCMP process to remove excessive dielectric material above thesemiconductor mesa 120, and etching back to recess the dielectricmaterial to reach the desired thickness range.

In the present embodiment, the TFET isolation layer 130 and the TFETisolation 150 both include silicon oxide and are collectively labeledwith numeral 150 in FIG. 7.

Referring to FIGS. 8 and 12, the method 200 includes an operation 216 toremove a portion of the gate material stack uncovered by the TFETisolation layer 150. The operation 216 includes an etch process toselectively etch the gate material layers in the top portion of the gatematerial stack. The etch process may include more than one steps tunedto etch respective gate material layers. By removing the top portion ofthe gate material stack, the gate of the corresponding TFET is formed onthe sidewall of the middle portion of the first semiconductor mesa 120Awith a horizontal extending portion for contact.

Referring to FIGS. 9 and 12, the method 200 includes an operation 218 toform a source 152 of the TFET device in the first semiconductor mesa120A. In the present embodiment, the source 152 is formed in the topportion of the first semiconductor mesa 120A. Particularly, the drain126 has a first type conductivity and the source 152 has a second typeconductive that is opposite from the first type conductivity. Forexample, if the first type conductivity is n-type (or p-type), thesecond type conductivity is p-type (or n-type). In one embodiment wherethe TFET is n-type, the drain 126 includes a n-type dopant (such asphosphorous) and the source 152 includes a p-type dopant (such asboron). In another embodiment where the TFET is p-type, the drain 126includes a p-type dopant and the source 152 includes a n-type dopant. Achannel 154 is defined in the middle portion of the first semiconductormesa 120A.

In one embodiment, the operation 218 includes removing the hard mask116, forming a patterned photoresist layer on the TFET isolation layer156, performing the ion implantation process using the patternedphotoresist layer as an implantation mask, and removing the patternedphotoresist layer 156 thereafter. The patterned photoresist layer 156has an opening configured such that the first semiconductor mesa 120A isuncovered by the patterned photoresist layer. During the ionimplantation, the TFET isolation layer 150 serves as an implantationmask in addition to the patterned photoresist layer 156 so that only thetop portion of the first semiconductor mesa 120A is implanted thereby.

In yet another embodiment, the operation 218 further includes recessingthe first semiconductor mesa 120A and epitaxy growing on the recessedsemiconductor mesa 120A with a semiconductor material that is same tothat of the semiconductor substrate 110 (such as silicon) or different(such as silicon germanium). Dopant of the source 152 may be introducedby an ion implantation by in-situ doping. In the in-situ doping, theepitaxy growth includes a precursor having the corresponding dopantchemical so that the dopant is simultaneously formed during the epitaxygrowth. This method may achieve a high doping concentration of thesource 152. In a particular example, the operation 218 includes removingthe hard mask 116, recessing a portion of the semiconductor mesa 120A byan etch process, and epitaxy growing on the recessed semiconductor mesawith in-situ doping. According to one embodiment, by recessing andepitaxy growth, thus formed source 152 has a smoother interface betweenthe source and the channel. Furthermore, the corresponding junction hasan enhanced performance. When a different semiconductor material isepitaxy grown for the source, a proper strain effect may be generatedfor the enhanced mobility and device performance.

The operation 218 may further include an annealing process to anneal thesource 152 for activation. The annealing process may be implementedright after the corresponding ion implantation (or epitaxy growth) or isalternatively implemented after the formation of other doped featuresfor collective activation. In various examples, the annealing processincludes RTA, laser annealing, spike annealing, MSA, or other suitableannealing technique.

The channel 154 is defined between the source 152 and the drain 126.Particularly, the channel 154 is defined in the middle portion of thefirst semiconductor mesa 120A. The channel 154 is vertically configuredso that the current of the TFET vertically flows through the channel 154from the source 152 to the drain 126.

In one embodiment, the channel 154 is neutral (un-doped). In anotherembodiment, the channel is lightly doped. In one example, the channel154 has a conductivity type same to the conductivity type of the drain126. For instance, the channel has a n-type dopant when the TFET isn-type, or the channel has a p-type dopant when the TFET is p-type. Inthis case, the doping concentration of the channel 154 is substantiallyless than that of the drain 126.

In the present embodiment, the source 152 has a small overlap with thegate stack of the TFET to ensure that the channel 154 is completelycoupled with and controlled by the gate stack.

Referring to FIGS. 10 and 12, the method 200 includes an operation 220to perform an ion implantation 158 to form the drain pickup feature 160of the TFET. The drain pickup feature 160 is formed in a top portion ofthe second semiconductor mesa 120B. The drain pickup feature 160 has asame type of conductivity as that of the drain 126 and is contacted withthe drain 126 but has a doping concentration greater than that of thedrain 126 to reduce the contact resistance. In the present embodiment,the drain pickup feature 160 includes a n-type dopant (such asphosphorous) when the TFET is n-type or a p-type dopant (such as boron)when the TFET is p-type.

In one embodiment, the operation 220 includes forming a patternedphotoresist layer 162 to cover the first semiconductor mesa 120A in thefirst region 112; and performing the ion implantation 158 to the secondsemiconductor mesa 120B using the patterned photoresist layer 162 as animplantation mask, and removing the patterned photoresist layerthereafter by wet stripping or plasma ashing.

The drain pickup feature 160 formed by the ion implantation 158 may befurther annealed for activation by an annealing process. The annealingprocess may be implemented after the ion implantation 158 in theoperation 220 or is alternatively implemented after the formation ofother doped features for collective activation.

Referring to FIGS. 11 and 12, the method 200 may further include anoperation 222 to form various contacts to the TFET. In the presentembodiment, the contacts 163, 164 and 165 are formed in an interlayerdielectric (ILD) 166. The contact 163 is configured to land on the firstsemiconductor mesa 120A and is electrically connected to the source 152.The contact 164 is configured to land on the horizontal portion of thegate and is electrically connected to the gate. The contact 165 isconfigured to land on the second semiconductor mesa 120B and iselectrically connected to the drain pickup feature 160, therefore iselectrically connected to the drain 126. Particularly, the sourcecontact 163 and the drain contact 165 have a same height since the drainhas a raised structure such that the source and drain are in the samehorizontal level.

In FIG. 11, the ILD 166 collectively refers to the dielectric materiallayers that include the TFET isolation layer 130 and the TFET isolationlayer 150 and further include a dielectric material layer deposited onthe TFET isolation layer 150. The ILD 166 includes silicon oxide or alow k dielectric material or other suitable dielectric material. Invarious embodiment, the ILD 166 includes silicon oxide, silicon nitride,silicon oxynitride, polyimide, spin-on glass (SOG), fluoride-dopedsilicate glass (FSG), carbon doped silicon oxide, low-k dielectricmaterial, and/or other suitable materials. The ILD 166 may be formed bya technique including spin-on, CVD, sputtering, or other suitableprocesses.

The contacts are conductive components in the interconnect structure andprovide electrical routing between the devices and the metal line in thevertical direction. In one embodiment, the operation 222 includesdepositing a dielectric material layer for the ILD, performing a CMPprocess to planarize the ILD, forming a patterned mask layer having aplurality of openings to define the regions for the contacts, etching toform the contact holes using the patterned mask layer as an etch mask,filling a conductive material in the contact holes, and performinganother CMP process to remove the excessive conductive material formedon the ILD. The patterned mask layer may be a patterned hard mask layeror alternatively a patterned photoresist layer. The patterned hard masklayer is similar to the patterned hard mask 116 in terms of formationand composition. The formation of the patterned photoresist layer issimilar to that of the other patterned photoresist layers previouslydescribed. The conductive material of the contacts includes metal, metalalloy or other suitable conductive material. In the present embodiment,the conductive material of the contacts includes tungsten (W). Thecontacts may further include other material. For example, the contactsinclude a lining layer, such as titanium nitride or tantalum nitride,formed on the sidewalls of the contact holes before the filling of theconductive material to the contact holes. The filling of the conductivematerial in the contact holes may use a suitable technology, such as CVDor plating.

The operation 222 may further includes forming other interconnectfeatures and other fabrication steps (such as passivation) in thebackend of the line. The interconnect structure includes horizontalconductive features (metal lines) and vertical conductive features (suchas vias and contacts). The interconnect structure includes conductivematerials such as aluminum, aluminum/silicon/copper alloy, titanium,titanium nitride, tungsten, polysilicon, metal silicide, orcombinations, being referred to as aluminum interconnects. Aluminuminterconnects may be formed by a process including physical vapordeposition (or sputtering), chemical vapor deposition (CVD), orcombinations thereof. Other manufacturing techniques to form thealuminum interconnect may include photolithography processing andetching to pattern the conductive materials for vertical (via andcontact) and horizontal connects (conductive line). Alternatively, acopper multilayer interconnect may be used and include copper, copperalloy, titanium, titanium nitride, tantalum, tantalum nitride, tungsten,polysilicon, metal silicide, or combinations. The copper multilayerinterconnect may be formed by a technique such as CVD, sputtering,plating, or other suitable process. The metal silicide used inmultilayer interconnects may include nickel silicide, cobalt silicide,tungsten silicide, tantalum silicide, titanium silicide, platinumsilicide, erbium silicide, palladium silicide, or combinations thereof.

Other fabrication steps may be implemented before, during and after theoperations of the method 200.

Thus formed semiconductor structure 100 includes a vertical TFET and mayfurther include another device integrated with the vertical TFET in acircuit. In one embodiment, the source and drain are leveled, the sourcecontact and the drain contact have a same height, the source and draincontacts can be formed in a same procedure with less fabrication costand improved performance and reliability. In another embodiment, theraised drain structure may be used as a resistor, as various embodimentsdescribed below.

The method 200 and the semiconductor structure 100 made thereby aredescribed above in various embodiments. However, the present disclosuremay include other alternatives and modifications. For example, thesource and drain are switched such that the drain is formed on the topportion of the first semiconductor mesa 120A, and the source is formedon the bottom portion of the first semiconductor mesa 120A and isextended to the second semiconductor mesa 120B through the semiconductorsubstrate 110.

FIG. 13 is a sectional view of a semiconductor structure 230 constructedaccording to another embodiment of the present disclosure. Similardescriptions (including features and operations to form the features)are eliminated for simplicity. The semiconductor structure 230 includesa vertical TFET in the first active region 232 and a resistor in thesecond active region 234. In this embodiment, the vertical TFET issimilar to the one in FIG. 11 and the resistor in the second activeregion 234 is formed in a third semiconductor mesa 120C in the secondactive region 234 and is further extended to the semiconductor substrate110. Particularly, the resistor has a continuous doped feature 236including a vertical portion formed in the third semiconductor mesa 120Cand a horizontal portion formed in the semiconductor substrate 110. Theresistor is a two terminal passive device with two contacts 240 and 242contacting to the two ends of the resistor. The contact 240 lands on thesemiconductor substrate 110 and contacts to one end of the doped feature236. The contact 242 lands on the third semiconductor mesa 120C andcontacts to another end of the doped feature 236.

The semiconductor structure 230 is formed by a method similar to themethod 200. In one embodiment, the doped feature 236 is formed by theoperation 208 to form the drain 126. In one example, the hard mask 116on the third semiconductor mesa 120C is removed and the operation 208 isexecuted thereafter so that the corresponding implantation process 124introduce the dopant into the top portion of the third semiconductormesa 120C as well and the doped features 236 is thus extended from thetop surface of the third semiconductor mesa 120C to the semiconductorsubstrate 110 within the second active region 234.

In another embodiment, the doped feature 236 is formed in the operation220 to form the drain pickup feature 160. In this case, the TFETisolation layers 130 and 150 are patterned in the correspondingoperations such that the second active region 234 is not covered by theTFET isolation layers.

In another embodiment, the doped region 236 is formed by a separate ionimplantation. Since the doped feature 236 functions as a resistor, theresistance of the resistance thus can be tuned by the dopantconcentration.

As noted above, two contacts 240 and 242 are formed on two sides of thedoped feature 236. In other embodiment, more contacts are formed on twosides of the doped feature 236. For example, multiple contacts areformed on the left side of the doped feature 236 and are configuredalong the left side, serving as a first terminal of the resistor.Multiple contacts are formed on the right side of the doped feature 236and are configured along the right side, serving as a second terminal ofthe resistor.

FIG. 14 is a sectional view of a semiconductor structure 250 constructedaccording to another embodiment of the present disclosure. Thesemiconductor structure 250 includes a plurality of resistors connectedin series. FIG. 14 only shows three resistors for illustration. Eachresistor is similar to the resistor of FIG. 13 and is formed by the samemethod. The semiconductor structure 250 includes a plurality of STIfeatures 122 formed in the semiconductor substrate 110, defining aplurality of active regions, such as active regions 252, 254 and 256 inthe present example. Each active region includes a semiconductor mesa,such as semiconductor mesas 120C, 12D and 120E in the active regions252, 254 and 256, respectively. Each active region includes a resistorhaving a doped feature 236 formed in the corresponding semiconductormesa and extended to the semiconductor substrate 110 in thecorresponding active region. In the present example, the semiconductorstructure 250 includes a first resistor, a second resistor and a thirdresistor associated with the first, second and third semiconductor mesas(120C, 120D and 120E), respectively.

Each resistor is connected to the two terminals: a first contact 258landing on the semiconductor substrate as a first terminal and a secondcontact 260 landing on the respective semiconductor mesa as a secondterminal.

Furthermore, the semiconductor structure 250 includes various conductivefeatures 262 in the interconnect structure. The conductive features 260may include metal lines and via features configured to couple theplurality of the resistors in series. Particularly, the second contact260 of the first resistor is electrically connected to the first contact258 of the second resistor. The second contact 260 of the secondresistor is electrically connected to the first contact 258 of the thirdresistor, and so on (if more resistors are in series connection). Thusintegrated resistor is a two terminal passive device. In this example,the first contact 258 of the first resistor serves as a first terminaland the second contact 260 of the third resistor serves as a secondterminal.

FIG. 15 is a sectional view of a semiconductor structure 270 constructedaccording to another embodiment of the present disclosure. Thesemiconductor structure 270 includes a plurality of resistors connectedin parallel. FIG. 15 only shows three resistors for illustration. Eachresistor is formed in a semiconductor mesa.

The semiconductor structure 270 includes various STI features 122 formedin the semiconductor substrate 110, defining an active region. Multiplesemiconductor mesas are formed on the semiconductor substrate 110 withinthe active region. In the present embodiment, exemplary threesemiconductor mesas 120F, 120G and 120H are formed on the active region.multiple resistors are formed on the semiconductor mesas, respectively.In the present example, the semiconductor structure 250 includes a firstresistor, a second resistor and a third resistor associated with thefirst, second and third semiconductor mesas (120F, 120GD and 120H),respectively.

Each resistor includes a doped feature 236 formed in the correspondingsemiconductor mesa, extended to the semiconductor substrate 110. Thedoped features 236 are merged together in the semiconductor substrate110. Each resistor has two terminals: a first contact 258 landing on thesemiconductor substrate 110 as a common contact to the resistors in theactive region; and a second contact 260 landing on the respectivesemiconductor mesa as a second terminal.

Accordingly, the three resistors are coupled together to form a twoterminal passive device: the first contact 258 as a first terminal andthe second contacts 260 electrically connected together through theconductive features 262 to form a second terminal.

Thus, the present disclosure provides one embodiment of a semiconductorstructure. The semiconductor structure includes a semiconductorsubstrate having a first region and a second region; a firstsemiconductor mesa formed on the semiconductor substrate within thefirst region; a second semiconductor mesa formed on the semiconductorsubstrate within the second region; and a field effect transistor (FET)formed on the semiconductor substrate. The FET includes a first dopedfeature of a first conductivity type formed in a top portion of thefirst semiconductor mesa; a second doped feature of a secondconductivity type formed in a bottom portion of the first semiconductormesa, the second semiconductor mesa, and a portion of the semiconductorsubstrate between the first and second semiconductor mesas; a channel ina middle portion of the first semiconductor mesa and interposed betweenthe source and drain; and a gate formed on sidewall of the firstsemiconductor mesa.

The present disclosure also provides another embodiment of asemiconductor resistor. The semiconductor resistor includes asemiconductor substrate having a first active region; a firstsemiconductor mesa formed on the semiconductor substrate within thefirst active region; a first resistor formed on the semiconductorsubstrate within the first active region; and a first contact and asecond contact connected to two ends of to the first resistor,respectively. The first resistor includes a first doped feature formedon the first semiconductor mesa and extended to the semiconductorsubstrate within the first active region. The first contact lands on thesemiconductor substrate and the second contact lands on the firstsemiconductor mesa.

The present disclosure also provides an embodiment of a method offorming a tunnel field effect transistor (TFET). The method includesforming a first semiconductor mesa and a second semiconductor mesa on asemiconductor substrate; performing a first implantation to form a drainof a first type conductivity, wherein the drain is a continuous dopedfeature extended from the first semiconductor mesa to the secondsemiconductor mesa through the semiconductor substrate; forming a firstdielectric layer on the semiconductor substrate and sidewall of thefirst and second semiconductor mesas; forming a gate stack on thesidewall of the first semiconductor mesa and extending horizontally onthe first dielectric layer; forming a second dielectric layer on thefirst dielectric layer and a horizontal portion of the gate stack;removing a portion of the gate stack uncovered by the second dielectriclayer; and forming, on the first semiconductor mesa, a source having asecond type conductivity opposite to the first type conductivity.

The foregoing has outlined features of several embodiments. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor structure, comprising: a semiconductor substrate having a first active region; a first semiconductor mesa formed on the semiconductor substrate within the first active region; a first resistor formed on the semiconductor substrate within the first active region, wherein the first resistor includes a first doped feature formed on the first semiconductor mesa and extended to the semiconductor substrate within the first active region; and a first contact and a second contact connected to two ends of to the first resistor, respectively, wherein the first contact lands on the semiconductor substrate and the second contact lands on the first semiconductor mesa, wherein the first doped feature formed on the first semiconductor mesa extends from the second contact to the semiconductor substrate, wherein the first doped feature has a same type of conductivity extending from the second contact to the semiconductor substrate.
 2. The semiconductor structure of claim 1, wherein the semiconductor substrate includes a second active region separated from the first active region by a shallow trench isolation (STI) feature, and the semiconductor structure further includes: a second semiconductor mesa formed on the semiconductor substrate within the second active region; a second resistor formed on the semiconductor substrate within the second active region, wherein the second resistor includes a second doped feature formed on the second semiconductor mesa and extended to the semiconductor substrate within the second active region; and a third contact and a fourth contact connected to two ends of to the second resistor, respectively, wherein the third contact lands on the semiconductor substrate and the fourth contact lands on the first semiconductor mesa, and the second resistor is connected with first resistor in series.
 3. The semiconductor structure of claim 2, further comprising an interconnect feature configured to electrically connect the second and third contacts.
 4. The semiconductor structure of claim 1, wherein the semiconductor substrate has the conductivity type, and wherein the first doped feature has a doping concentration greater than that of the semiconductor substrate.
 5. A semiconductor device, comprising: a substrate having a first region and a second region; a first mesa formed on the substrate within the first region; a second mesa formed on the substrate within the second region; and a field effect transistor (FET) formed on the substrate, wherein the FET includes a first feature of a first conductivity type formed in a first portion of the first mesa; a second feature of a second conductivity type formed in a second, different portion of the first mesa, the second mesa, and a portion of the substrate between the first and second mesas, wherein one of the first and second features is a source and the other of the first and second features is a drain; a channel in a middle portion of the first mesa and interposed between the source and drain; a gate formed on sidewall of the first mesa, a drain pickup feature formed in a top portion of the second semiconductor mesa, the drain pickup feature includes a doping species of the second conductivity type, and the drainpickup feature has a doping concentration different than that of the drain.
 6. The semiconductor device of claim 5, wherein the second conductivity type is opposite to the first conductivity type, the FET is a vertical tunnel field effect transistor (TFET), the first feature is a source of the vertical TFET, and the second feature is a drain of the vertical TFET.
 7. The semiconductor device of claim 6, wherein the second feature is a continuous feature extending from the first mesa to the second mesa through the substrate.
 8. The semiconductor device of claim 6, wherein the first mesa has a first top surface, the second mesa has a second top surface, and the first and second top surfaces are coplanar.
 9. A semiconductor structure, comprising: a semiconductor substrate having a first region and a second region; a first semiconductor mesa formed on the semiconductor substrate within the first region; a second semiconductor mesa formed on the semiconductor substrate within the second region; and a field effect transistor (FET) formed on the semiconductor substrate, wherein the FET includes: a first doped feature of a first conductivity type formed in a top portion of the first semiconductor mesa, wherein the first doped feature is a source of the FET; a second doped feature of a second conductivity type formed in a bottom portion of the first semiconductor mesa, the second semiconductor mesa, and a portion of the semiconductor substrate between the first and second semiconductor mesas, wherein the second doped feature is a drain of the FET, wherein the second conductivity type is opposite to the first conductivity type; a channel in a middle portion of the first semiconductor mesa and interposed between the source and drain; and a gate formed on sidewall of the first semiconductor mesa; and a drain contact that is connected to the drain and lands on the second semiconductor mesa; a source contact that is connected to the source and lands on the first semiconductor mesa; and a gate contact that lands on a horizontal portion of the gate and is configured between the source and drain.
 10. The semiconductor structure of claim 9, wherein the FET is a vertical tunnel field effect transistor (TFET).
 11. The semiconductor structure of claim 9, wherein the second doped feature is a continuous feature extending from the first semiconductor mesa to the second semiconductor mesa through the semiconductor substrate.
 12. The semiconductor structure of claim 9, wherein the first semiconductor mesa has a first top surface, the second semiconductor mesa has a second top surface, and the first and second top surfaces are coplanar.
 13. The semiconductor structure of claim 9, wherein the top portion of the first semiconductor substrate includes a semiconductor material different from that of the bottom portion of the first semiconductor mesa.
 14. The semiconductor structure of claim 9, wherein the vertical TFET further includes a drain pickup feature formed in a top portion of the second semiconductor mesa, the drain pickup feature includes a doping species of the second conductivity type, and the drain pickup feature has a doping concentration greater than that of the drain.
 15. The semiconductor structure of claim 9, wherein each of the first and second semiconductor mesas has a sidewall profile with a tilt angle in a sectional view, wherein the tilt angle is measured relative to a top surface of the semiconductor substrate, and the tilt angle is less than 90° and greater than 45°.
 16. The semiconductor structure of claim 9, wherein each of the first and second semiconductor mesas has a round shape in a top view.
 17. The semiconductor structure of claim 9, wherein the gate includes a gate dielectric layer having a high k dielectric material; and a gate electrode having a metal.
 18. The semiconductor structure of claim 9, further comprising: a third semiconductor mesa formed on the semiconductor substrate in a third region; a resistor having a doped featured formed in the third semiconductor mesa and extended to the semiconductor substrate; and a first contact and a second contact connected to two ends of to the resistor, respectively.
 19. The semiconductor structure of claim 18, wherein the first contact lands on the semiconductor substrate and the second contact lands on the third semiconductor mesa.
 20. The semiconductor structure of claim 18, further comprising a plurality of shallow trench isolation (STI) features formed in the semiconductor substrate and defining a first active region and a second active region, wherein the first and second semiconductor mesas are disposed on the first active region, and the third semiconductor mesa is disposed on the second active region. 